“For a long time, they didn’t have a vaccine for chickenpox—it was a childhood disease, and almost everybody caught it. Only that was a good thing back then, because most kids get over the chickenpox pretty easy. They itch for a week and then they’re fine. Better than fine. Having the virus once makes them resistant to catching it again, and for adults, chickenpox is no laughing matter. It can cause permanent nerve damage, severe scarring, all sorts of nasty side effects. Dr. Abbey looked placidly at Kelly. “People used to have chickenpox parties, where they’d deliberately expose their young children to somebody who was already sick.”

“That’s disgusting,” said Becks.

“Now that we have a vaccine, sure. Back then, it was a way to save your children from suffering a lot more. It wasn’t safe—kids died of chickenpox—but it was a damn sight better than the alternative.”

“I don’t understand,” I said.

I do, said George, very quietly.

“I do,” echoed Alaric. I turned to look at him, and he said, “When infants are exposed to live Kellis-Amberlee, they can’t amplify, but they still get sick. Only they can get better, can’t they? They can actually recover from the virus.”

“Bingo.” Dr. Abbey, touching her nose with her left index finger while she pointed at Alaric with her right. “Princess CDC, tell the nice man what he’s won!”

Kelly was silent.

I swallowed away the dryness in my throat, and said, quietly, “Please.”

My voice seemed very loud in the enclosed space of the lab. Kelly turned to face me and said, “Yes, sometimes early exposure can lead to individuals successfully fighting off a live Kellis-Amberlee infection. It’s impossible to run a standard blood test on an infant, because they can’t amplify, so we can’t find the usual amplification markers. But they’ll get sick. It’s been seen. And then, after a little while, they aren’t sick anymore.” Kelly stopped, choosing her next words with care: “Most of the individuals who undergo a potential infectious episode as infants develop one of the reservoir conditions when they get older, because their immune systems are preconditioned to respond.”

“Their bodies remember that the virus is bad, and they set up their own little kennels, filled with their own little packs of domesticated viral bodies,” clarified Dr. Abbey, leaning down to thump Joe on the side. He looked up at her adoringly, tongue lolling. “That’s what humanity does when faced with wolves. We take them in, tame them, and teach them how to keep us safe.”

“Yes,” agreed Kelly. “The reservoir conditions are a marker that the immune system has learned it needs to fight back when Kellis-Amberlee starts taking over.”